| Ecosystem water-use efficiency?WUE?is the key index of carbon water coupling process in the terrestrial ecosystem,which reflects the drought resistance ability of vegetation and the productivity level of the ecosystem.It is of great significance to study the adaptability and productivity level of vegetation.Poplar plantation has the largest planting area in China,and is widely used in timber forest,shelter forest system construction and plain greening in"Three North"area.Its dry matter accumulation is rapid but its transpiration is strong,while the ecology stability in northern China is fragile,so the poplar forest often has different degrees of ecological drought thus inducing various kinds of diseases and insect pests.Therefore,it is necessary to study the WUE of poplar plantation in order to clarify its impact law,which will help to guide the local water allocation to alleviate the ecological drought and achieve sustainable management.In this paper,flux observation technology and micro meteorological observation method were used to continuously observe the carbon water flux and microclimate of poplar plantation ecosystem in Shunyi from 2014 to 2017,and WUE was calculated by the ratio of gross ecosystem productivity GEP to evapotranspiration.In this paper,Pearson correlation,partial correlation and regression analysis were used to investigate the effects of leaf area index?LAI?,precipitation?P?,PAR,VPD,VWC5,Ta,and canopy conductance on WUE?GPP,ET?at different time scales.The results show that:?1?The average WUE of the poplar plantation ecosystem in Beijing Shunyi Gongqing Forest Farm from 2014 to 2017 is about 2.83±0.56 g C·m-2d-1,which is significantly lower than that of some Canadian poplar forests and other temperate forests.The forest farm can improve WUE by introducing suitable and excellent poplar trees.?2?The dynamic changes of WUE,carbon and water fluxes and environmental biological factors of poplar plantation ecosystem in different time scales have some certain rules.The environmental conditions of poplar plantation,such as light,temperature and water,are generally better in the middle growing season from June to September.The average values of gross productivity and evapotranspiration are higher and WUE is lower in the same period,which are about 10.43±2.96 g C·m-2d-1,4.58±1.38 mm·d-1 and 2.43±0.72 g C·kg-1H2O,respectively.The average value of LAI is about2.68±0.41 during that period.?3?Pearson correlation analysis showed that the dynamic change of WUE monthly value in the growing season of poplar plantation was mainly affected by the biological factor LAI and showed a significant negative correlation.Pearson correlation coefficient was-0.384,and WUE was not significantly affected by environmental factors.?4?The main influencing factor of WUE in the middle growing season of poplar plantation is the environmental factor PAR,which is significant negative patial correlation,and the partial correlation coefficient is-0.337.?5?Regression analysis also showed that the most important influencing factor of WUE in the mid-growing season of the poplar plantation was the environmental factor PAR,which had a significant negative correlation.The coefficient of determination R2 was 0.1783,and the biological factor Gc also had a significant negative correlation with WUE.There are some differences in the effect rules of PAR,VPD,Ta,Gc and other environmental or biological factors on GEP and ET,which lead to the complex dynamic changes of WUE.When PAR and Gc increase,they mainly increase ET and at last reduce WUE,while VWC5 has no significant effect on WUE.?6?When VPD?1.5 k Pa,the WUE mean value is 2.43±0.75 g C·kg-1H2O,which is significantly higher than the WUE mean value of VPD>1.5 k Pa,which is 2.02±0.49 g C·kg-1H2O.To sum up,the WUE of poplar plantation ecosystem in Shunyi still needs to be improved.The longer time scale WUE is mainly determined by its own biological factor LAI,while the shorter time scale WUE is mainly determined by the environmental factor PAR and also affected by the biological factor Gc to a certain extent.Further study on the effect of environment and biological factors on WUE,GEP and ET of poplar plantation will improve the poplar plantation water use efficiency of Three North area. |
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